Description of the Prior Art
Piezoelectric crystal devices are used primarily for precise frequency control and timing, with quartz being the piezoelectric crystal used in most applications. A quartz crystal acts as a stable mechanical resonator, which by its piezoelectric characteristics and high Q value can determine the frequency generated in an oscillator circuit. Of the four basic types of crystal oscillators known, the Oven Controlled Crystal Oscillator ("OCXO") is of particular interest in the present invention.
Until now, the atomic frequency standard exhibited superior stability to the crystal oscillator in the areas of frequency vs. temperature characteristics and aging. Atomic frequency standards have been well known for many years. They provide the highest standard of accuracy heretofore available. While atomic frequency standards embodied in the atomic clock have been commercially available for some time and have undergone many improvements, they still suffer from a number of disadvantages, including being cumbersome due to their large size, and having high power requirements, limited temperature range and, particularly in the case of beam type standards, a limited life.
The method of the present invention provides frequency vs. temperature ("f vs. T") stabilities for an OCXO which are from 100 to 10,000 times greater than presently available. The f vs. T stability of an OCXO depends on a number of factors, including (1) the static and dynamic f vs. T characteristics of the resonator in the sustaining circuit; (2) the difference between the oven's set-point and the point where the static f vs. T characteristic has a zero slope; (3) the oven's temperature excursions from the set-point or oven "cycling" range, and (4) the rate of change of temperature during the oven's temperature excursions.
Those concerned with the development of crystal oscillators, as well as atomic frequency standards, have long recognized the need for higher f vs. T stability. The present invention provides for a method of increasing the f vs. T stabilities of an OCXO which meets this long-felt need and suffers from none of the disadvantages of previously available crystal oscillators with a lesser f vs. T stability such as temperature-compensated crystal oscillators, prior art OCXO's and rubidium frequency standards.
In general, the method of the present invention encompasses the steps of making an SC-cut quartz resonator with turnover temperatures at or near the inflection temperature, inserting the resonator into a high-stability oscillator circuit, placing the resonator in a high-stability, high thermal gain oven and adjusting the oven temperature to a set-point at or near one of the turnover temperatures to provide for higher f vs. T stability in an OCXO.
The temperature stable OCXO produced in accordance with method of the present invention would be extremely useful in applications where recalibration capability, such as that attained with the aid of a Global Positioning System ("GPS") is frequently, but not continuously available. An example is a mobile communications operation where the user returns to a base station which has a recalibration facility where the OCXO can be automatically recalibrated while batteries are being recharged. Such a temperature-stable OCXO along with a recalibration system would be more advantageous in certain situations than having an OCXO continuously locked onto GPS.
The prior art in this field may be found in the following references:
J. A. Kusters, "The SC-Cut Crystal-An Overview," Proceedings of IEEE Ultrasonics Symposium, 1981, pp. 402-409;
E. P. EerNisse and J. A. Kusters, "Orientation Dependence of `True` SC-Cuts," Proceedings 44th Annual Symposium on Frequency Control, 1990, pp. 185-192;
J. R. Vig and F. L. Walls, "Fundamental Limits on the Frequency Instabilities of Quartz Crystal Oscillators," Proceedings IEEE Frequency Control Symposium, 1994, published Oct. 1994;
C. A. Adams, D. C. Bradley & J. A. Kusters, "X-Ray Technology-A Review," Proceedings 41st Annual Symposium on Frequency Control, 1987, pp. 249-257;
J. R. Vig, W. Washington & R. L. Filler, "Adjusting the Frequency vs. Temperature Characteristics of SC-Cut Resonators by Contouring," Proceedings of the 35th Annual Symposium on Frequency Control, 1981, pp. 104-109;
R. L. Filler and J. R Vig, "Resonators for the Microcomputer-Compensated Crystal Oscillator," Proceedings of the 43rd Annual Symposium on Frequency Control, 1989, pp. 8-15;
F. L. Walls, "Analysis of High Performance Compensated Thermal Enclosures," Proceedings 41st Annual Symposium on Frequency Control, 1987, pp. 439-443;
J. A. Kusters, et. al., "A No-Drift and Less than 1.times.10.sup.-13 Long Term Stability Quartz Oscillator Using a GPS S/A Filter," Proceedings 1994 IEEE International Frequency Control Symposium, 1994, published Oct. 1994; and
J. R. Vig, U.S. Pat. Ser. No. 4,375,604 entitled "Method of Angle Correcting Doubly Rotated Crystal Resonators," Mar. 1, 1983.